1. Field of the Invention
The present invention relates generally to testing and evaluating different wound treatment variables.
2. Description of the Related Art
A. External Factors
In the field of wound care, positive healing outcomes are facilitated by a variety of both internal (i.e. patient-subject) and external factors. External factors of interest include: 1) bacterial control, 2) moisture control, and 3) surface contact properties. These and other factors can be manipulated, and the results evaluated, by the system and method according to the present invention, which is disclosed herein. The system and method of the present invention can be applied in a bench-top or an in-vitro manner and also on the external or skin surface of an in vivo animal experimental model.
Properties such as bacterial adherence—the number of bacteria that will adhere to a dressing and the difficulty or ease of removing them by elution techniques—and the growth rates of bacteria in or on a particular dressing material, when compared to rate of growth or doubling time of bacteria in or on a culture medium alone, have direct clinical application. Important considerations include whether bacteria will move up or into a dressing placed on a wound that is contaminated or infected, whether bacteria can be washed away from the field by irrigation through the dressing, and whether bacteria growth and toxic behavior is impeded or enhanced once bacteria are in the dressing.
Properties of moisture control—such as the retention of drainage or moisture versus the loss of moisture from the wound surface by rapid removal to the point of drying—can be assessed in bench top models using such parameters as vapor transmission rates and time to dryness for measured amounts of solutions, such as saline, pure water, colloids and mixtures. Various surfaces can be employed, from porous material mimicking dermis and fibrous wounds (e.g. chamois) to non-porous materials mimicking keratins and other tissue layers. These can be hydrophobic or hydrophilic, absorbent (gels) or non-absorbent materials. Classically, wood and plastic surfaces exhibit very different moisture handling characteristics when used with the same cover dressing material.
Surface contact properties can be predicted from an examination of the surface characteristics of the dressing material and the known behavior of the material. For example, a magnified surface scan of the material shows “pore size.” This is known to correlate to both foams and fiber weaves with the ability to “pass through” or retain moisture and drainage. Pore size also correlates to whether the dressing allows ingrowth of granulation tissue (even to the point of incorporation of the dressing layer), which prevents or greatly impedes the ability of the epithelial layer to migrate across the open wound and create “closure” or “healing,” or whether the pore size is small enough to prevent ingrowth of granulation and thus favors epithelial migration beneath the dressing. The surface contact properties also influence moisture retention properties and drying properties such that the resulting scab formation, or lack thereof, will either dry into the dressing and become part of it, requiring the epithelium to migrate beneath and through this layer impeding this migration, or it will form an extremely thin scab with moist wound healing properties such that the epithelial migration “streams” readily across the wound, e.g., as disclosed in G. D. Winter U.S. Pat. No. 4,373,519. Thus, if the drying scab is bound to the dressing, its removal can be disruptive or even destructive of the epithelial layer growing beneath it.
Surface contact properties are intimately related to moisture control properties, which, in turn, are intimately related to bacterial control. So, an analysis of single properties can be associated with effects in all three factors of concern. Bench top and/or in vitro studies can give information about all three factors.
B. Bacterial Adherence
Using test tubes with growth or culture media and measured amounts of bacteria (total count or per unit amount), measured amounts of dressing material to be studied can be inserted into the media for specified amounts of time and then removed to measure the amount of bacteria on the material and the residual bacteria in the test tube. Even short or acute time periods are clinically meaningful and can be used to decrease the confounding factor of bacterial doubling during the time of the experiment, but prolonged incubation periods can also reveal much about the interaction of bacteria and the dressing material over time.
A method to test total submersion of materials that may be hydrophobic, or lightweight and tending to “float” and not submerge if left on their own, is to attach a screen to a glass rod (or rod made of the same material as the test tube or container) or to simply expand and flatten the end of a glass rod itself so that it could push the material beneath the surface. The glass rod can be sized so that the desired position beneath the surface is obtained by fixing one end of the rod to the test tube cap so that when the cap is screwed down in place, the rod is the appropriate distance below the surface to force the material into submerged position. After the desired length of time, the material is removed in sterile fashion and a bacterial count is obtained from the residual fluid, giving the difference as the amount absorbed and adhered to the dressing material.
These factors (absorption vs. adherence) are separated by compressing the material against a screen, collecting the absorbed but now compressed out fluid and getting the bacterial count on that. The difference from the original total colony count is the amount adherent to the dressing. This can be confirmed directly by mincing the material thoroughly, plating widely, and doing a colony count or by section and microscopic or histologic staining and counting techniques of measured areas/volumes. Separation techniques such as sonication and elution can also be employed.
Once base lines are obtained, the amount of pressure for submersion—mimicking the compression produced by collapse of surface contact material in a negative pressure wound therapy (NPWT) dressing—can be varied. The immersion/submersion or floating contact can be done with or without agitation (shaking mechanically), centrifugation, sonication, and other mechanical factors. The dressing material(s) can be studied intact or minced to increase surface exposure to define the characteristics that belong to the material specifications and those that correspond to its shape.
C. Bacterial Elution
Once bacterial adherence properties of a material are known, the tightness and permanence, or strength, of the adherence can be tested by evaluating how easily the bacteria can be removed by rinsing or eluting the material. Irrigation, washing, and the simple outward flow of natural moisture and drainage up into an absorbent material having a dry or drying outer layer to ensure continuous upward or outward movement of moisture/drainage are standard clinical means used to remove bacteria from a wound surface. Rinsing or elution testing of the dressing material, once it is impregnated or contaminated with bacteria in a defined or known number, is an important parameter that needs to be measured and known to characterize the behavior of bacterial material. Differing amounts of various elution solutions (starting simply with physiological saline) tested over predetermined time periods can give information by examination and definition of the amount of bacteria in the elution and the amount in the material.
Clinically, a known standard therapy with proven efficacy is to fill a NPWT dressing with a given amount of saline, provide a “dwell time” in minutes, and then suction it out. The elution can be plated, if the amount is small, or run through a filter that would pick up bacteria and then count or plate the bacteria. The material itself can be handled as per the above discussion.
D. Bacterial Growth in Dressing
Once the bacterial absorption and adherence potential of a dressing is known, the subsequent behavior of the bacteria in the dressing material must be determined. Clinically, this determines how frequently a dressing combination needs to be changed or whether it can be left in place and for how long. For example, in the NPWT system by Acelity L.P. Inc. (formerly Kinetic Concepts, Inc.), San Antonio, Tex., there are two very different foam materials. One of the foam materials comes with the option of silver coating and also the option of an attached liner of different wicking and absorption properties. There are several liners or membranes used with the system and available from other sources that change the absorption/bacterial adherence properties and behavior of the system. Several NPWT systems are commercially available.
Once the material to be tested is contaminated or impregnated with bacteria (e.g., introduced on one edge only, introduced by immersion combinations, etc.), growth over time experiments can be performed and the analysis techniques developed above can again be utilized to determine the bacterial count in and on the material (e.g., after a specified number of days). This information is particularly significant because clinically there are questions about the behavior of bacteria over days in and on the material and at the material/wound interface if the physical environment is being changed with such factors and effects as intermittent negative pressure, negative pressure at different levels, irrigation or non-irrigation, etc. Specific inquiries include the effect on toxicity products from the bacteria and the effect on biofilm production.
E. Moisture
The most-used methods for specifying moisture transmission rates of materials are supported by well-established and published test equipment operating procedures and standards for industry and science, and those methods are preferably utilized for standard reporting of “Moisture Vapor Transmission Rates” (MVTR). Because of the multiple combinations of materials and dressings available for use with multiple different NPWT models, other methods to support a summative statement can be analyzed, in addition to the specifications of individual materials. Simple “gravimetric” methods of precisely weighing foam materials before and after use or application steps provide indications of the amount of absorbed and retained drainage. This also applies to in vivo testing. Since many of the in vivo test methods will employ NPWT, a vacuum or suction system that collects and condenses moisture and liquid drainage must be utilized, and simple measurements of this collection rate (corresponding to quantity/time) can be obtained.
This same approach can be applied to in vitro methods that are extended over time. For example, experiments to demonstrate bacterial adherence and elution properties in very short term experiments (e.g., minutes) are discussed above. Bacterial growth over time (e.g., days) also needs to be monitored, and in these experiments, the air above the plate or tube of growth media with implanted material or plated diced material should be controlled so that the aerobic/anaerobic concentration of the overlying gaseous milieu is known. This can be done with either a positive pressure inflow source to a contained growth chamber or with a negative pressure or vacuum source on the outflow side. This outflow vapor can then be “distilled” in cooled, coiled loops to collect moisture or analyzed for moisture vapor and gas use by the organisms. A system can be created which allows measurement of environmental changes reflecting the metabolic changes and uses of the growing organisms as opposed to just measuring bacterial growth as an endpoint. Humidity, temperature and other factors are controlled in the practice of the present invention.
Change in the material itself, in addition to weight gain, is another important consideration for wound treatment variables. Questions include whether magnified examination shows swelling of fibers and spaces; whether moisture is retained as liquid and not just vapor; and whether, for example, moisture or liquid moves by capillary action between relatively intact but closely packed fibers or whether there is absorption by swelling of the spaces between the fibers. Change in the media or material (e.g., tissue in the case of an in vivo application) in contact with the material being examined is also a consideration. This is perhaps easiest explained in the in vivo setting. For example, in pig back experiments, investigatory topics can include the amount of scabbing and dried protein bonding or adherence of the dressing to whichever surface or tissue it was on. A direct visual analog scale can be used to rate and record the amount of drying and moisture retention, from dry scab at one end of the scale to no adherence, moist conditions with no drying, or even moisture retention with visible liquid retention, at the other end of the scale. In vitro we can look for changes in the moisture appearance of the media in which the material is implanted or plated (e.g., does the presence of the material “dry out” the media compared to a control plate), and the volume of residual media and material can be measured after several days in comparison to initial measurements and control tubes. In addition, a dynamic in vitro system can be established in which the dressing is placed on a suitable non-reactive surface and “contained” by a NPWT drape with input and output ports and fluids carefully controlled and measured at each end with the dressing maintained as an active culture medium, bio-reactor, or cell or biochemical incubator.
F. Surface Contact
Dennis P. Orgill, M. D., Ph.D., in his writings on NPWT (e.g., Saxena, V. et al., Vacuum-Assisted Closure: Micro Deformations of Wounds and Cell Proliferation, Plastic and Reconstructive Surgery, (October 2004), pp. 1086-1096; Huang, C. et al., Effect of Negative Pressure Wound Therapy on Wound Healing, Current Problems in Surgery 51 (2014), pp. 301-331) makes the case that one of the mechanisms of action of NPWT is “micro and macro deformation.” The tissue in contact with the material deforms to match the surface of the material in contact. For example, Dr. Orgill described the deformation effect of polyurethane ether or the “black foam” (Granufoam brand) of Acelity L.P. Inc. To be a quilting-like effect of the compression by the reticules and the tension of the open pore spaces subjected to negative pressure resulting in a peak and valley tissue surface integrated into the foam which then blocks the ability of the epithelial layer to advance or migrate across this tissue surface.
NPWT can be accurately compared to a “vacuum press” such as those used in industry to seal laminates or veneers to underlying wood. These “press” conditions inside the NPWT dressing are such that any material is held in intimate forceful surface contact relationship to the tissue it is applied to. This results in the ability to augment or enhance the surface characteristics and effects of the material chosen for contact. Thus, one can see how important it is to evaluate the potential and actual surface contact effects of various dressing materials, particularly those that are placed as contact layers or surfaces in NPWT devices where these effects are enhanced by sub-atmospheric (vacuum) pressure.
The clinically understood (cf., Dr. Orgill) micro-deformation effect of open-cell reticulated foam is that this material presents to the surface as alternating spicule or reticule points which penetrate the surface to the branching of the reticules as they weave around and form the cells or pores of the material. That is, they penetrate under the compressive effects of the interior of the NPWT vacuum press to the point on the penetrating strand where the material is no longer an individual strand or vertically or perpendicularly oriented to the surface but is branching or horizontally oriented to the surface.
The degree of penetration can be controlled by the cell or pore size of the foam. The spicules alternate with spaces which communicate with the negative pressure in an open cell foam (the mechanism is different in closed cell material). Thus, the surface of contact with Granufoam material is presented with alternating points of compression and tension in “micro-array” resulting in mechanical induction or “mechano-transduction” of sensing systems sensitive to these forces in the cell. This in turn signals and initiates chemical and metabolic changes in the cell.
As the reporting of the surface feature of dressing material becomes predominantly more frequently expressed as “pore size” related, an old method and a differentiating feature of cotton fiber gauze seems to be becoming lost in this process. The important distinction historically made during the last century for the “pore size” equivalent for gauze is the weave or mesh size, e.g., the number of fibers in the weft and warp per unit area. Fine mesh gauze was originally defined as gauze with a “44/36” weave. That weave was empirically determined to be the cutoff point through which granulation would either grow up into the gauze or be blocked by the closeness of the weave and not penetrate the gauze, thereby allowing epithelialization to proceed beneath the gauze layer. So, by definition “fine mesh gauze” was that with a 44/36 weave and could be used as a “surface” dressing allowing (re)epithelialization beneath it. Coating the hydrophilic gauze with hydrophobic ointments and escharotics of all kinds produced a family of dressings that could produce any effect one desired on the wound surface.
Cotton gauze has largely been supplanted by silks, synthetics, and foams with all degrees of pore size, including microscopic so that the compressed foam surface presents like glass (e.g., Epigard), and with gels and colloids of all degrees of absorption, smoothness, and adhesiveness. The effects of a material on a wound surface can be augmented and enhanced with NPWT. The vacuum press effect also enhances dressing adherence and maintains close apposition over wound and body irregularities, even over large areas. These properties tend to enhance the wound-healing effects of the material.
G. Surface Contact Properties
Examination of the surface of a dressing, by all manner of optics and scanning (e.g., with a scanning electron microscope), can reveal the physical properties and dimensions of protrusions, indentations, and spaces. Pore size is a significant variable factor. It is understood that the surface of a dressing is connected to the interior, and the dressings contact properties are also tied up with the way it handles bacteria and moisture/vapor/drainage. Defining properties such as hydrophilic and hydrophobic relationships to moisture help to predict and understand a dressing's behavior. Defining its ability to wick or move moisture laterally is also important in understanding its function.